The objectives of this program are to study the structure of the tissues of the hematopoietic system in certain normal pathologic and experimental conditions using transmission electron microscopy, scanning electron microscopy, freeze-fracture etch, autoradiography and other morphologic and related techniques. We shall be studying such phenomena as the cooperation of macrophages and erythroblasts in erythropoiesis, the interaction of macrophages, T-lymphocytes and B-lymphocytes in plasma cell differentiation, the delivery of blood cells to the circulation by the bone marrow, the sorting of hematopoietic cells in the bone marrow and of lymphocytes in the periarterial lymphatic sheath of the spleen, the migration pathways of lymphocytes and monocytes, vascular patterns in spleen and bone marrow, the induction of differentiation of hematopoietic cells by fibroblastic cells of bone marrow and spleen, and the degranulation of sensitized basophils by the addition of antigen in vitro. Our experimental animal models include the aplastic anemia in Steel and Wv mice, the congenital spherocytic anemia in the deer mouse, the mobilization of reticulocytes in rats after phenylhydrazine and of normoblasts in rabbits after saponin, the experimental viral induced leukemias in the rat and in the cow and the naturally occurring leukemias and aplastic anemias in human beings.